The present invention relates to an optical apparatus which forms a plurality of images on a photoelectrical conversion element to perform focus detection in a phase difference detection method.
In single-lens reflex cameras, AF (auto-focus) in a TTL (Through the Taking Lens) phase difference detection method is generally employed. In the phase difference detection method, a luminous flux from an object is taken through an image-pickup lens and then reflected by a movable mirror and directed onto a surface (a first image-forming surface) equivalent to a light-receiving surface of a film or an image-pickup element. The luminous flux directed to the equivalent surface is split into two (pupil splitting) by a secondary image-forming optical system including a separator lens, and the two light components are directed to a pair of line sensors for AF. The difference between the two images (phase difference) on the pair of line sensors is detected to determine the amount of displacement of a focus lens in the image-pickup lens from the in-focus position.
A digital camera has an image-pickup element serving as a photoelectrical conversion element for photoelectrically converting an object image. When part of the image-pickup element is used as a sensor area for AF, it is not necessary to provide a line sensor dedicated to AF or a secondary image-forming optical system other than the image-pickup lens.
Thus, a focus detection system has been proposed in which part of an image-pickup element is used as a sensor area for AF, and two luminous fluxes separated by a split image prism included in an image-pickup optical system are directed to that area (Japanese Patent Laid-Open No. 2004-46132).
Another proposed structure includes a holographic optical element disposed closer to an object than a first image-forming surface to realize AF in the TTL phase difference detection method (Japanese Patent Laid-Open No. H04(1992)-147207).
In the method proposed in Japanese Patent Laid-Open No. 2004-46132 in which the split image prism is used, however, continuity of an image is essential on the border in the split image prism in contrast to the TTL phase difference detection method. For example, if the shape of image on the border in the split image prism is not a straight line, an out-of-focus state is determined even when an in-focus state is achieved. Such a constraint in focus detection makes it impossible to realize the same level of focus detection performance as that in the TTL phase difference detection method.
The method proposed in Japanese Patent Laid-Open No. H04(1992)-147207 in which the holographic optical element is used is similar to the TTL phase difference detection method in principle. However, the holographic optical element with large chromatic dispersion is used to form two images (AF images) in a direction of pupil splitting that are important in determining a focus state. Thus, wavelengths greatly affect a difference between incident angles of images having a phase difference that is important to providing high accuracy of focus detection. As a result, the method is not suitable for practical use when focus detection is performed.